The present invention relates to a control and/or data acquisition system for a pivotally actuated tracer, which system is adapted to provide compensation for the effects of gravity on the tracer and/or convert acquired data from a polar format to a Cartesian format.
Generally, tracers are used to acquire information about the shape of an object being traced (hereinafter "the object"). Such tracers typically have means (i.e., an object engager) for engaging the object, as well as means for moving the object engager along the object while the position of the object engager is monitored. The resulting position information then is used to determine the shape of the object.
Typically, the movement of the object engager is performed using linear actuators/position detectors. One linear actuator/position detector provides movement/position detection with respect to an X-axis, while another provides movement/position detection with respect to an orthogonal Y-axis. Such arrangements, while generally effective, leave room for improvement.
A tracer therefore has been developed by the Assignee hereof, which tracer realizes certain benefits by using at least one pivotally actuated object engager instead of a linearly actuated one. The exemplary tracer having a pivotally actuated object engager is described in a contemporaneously filed patent application entitled TRACER, CLAMP, AND OBJECT ENGAGER FOR HOLDING AND TRACING A LENS MOUNT OF AN EYEGLASS FRAME, A LENS, AND/OR A LENS PATTERN, TO RELIABLY DETECT A SHAPE THEREOF EVEN WHEN THE SHAPE INCLUDES WRAP-AROUND Ser. No. 09/270,115, filed on Mar. 16, 1999 by Andrews et al. on behalf of the Assignee hereof. The contents of that patent application are incorporated herein by reference.
Tracers having pivotally actuated object engagers achieve significant advantages over those which provide linear actuation of the object engager. Such pivot-based tracers, for example, can be provided using a relatively compact actuation mechanism. They also are more compatible with rotary encoders which provide positional information based on an actuating element's rotation. Rotary encoders typically are less expensive than linear encoders. The pivot-based tracer arrangements therefore achieve significant savings in manufacturing costs, as well as an advantageously compact structure.
The pivot-based tracer described in the aforementioned patent application is particularly well-suited for tracing lens mounts in an eyeglass frame. It also is well-suited for tracing of lenses or lens patterns.
Lens mounts, lenses, and lens patterns typically are traced in order to generate trace data, which data then is supplied to an edging apparatus. The edging apparatus then processes the edge of a lens blank to create an edge profile which matches the trace data. The resulting lens fits within the traced lens mount or matches the shape of the traced lens or lens pattern.
During a tracing operation, the object engager of the exemplary pivot-based tracer is actuated along the object to be traced. The object engager is associated with a pivot arm and has a predetermined object engaging feature (e.g., a stylus which engages an eyeglass frame, a groove for receiving the beveled edge of a lens, or a shoulder which engages an edge of a lens pattern). The object engaging feature is rotated along the inner circumference of the frame mount or around the outer circumference of the lens or lens pattern. During such rotations, the object engager is pivoted toward or away from the rotational axis and is extended or retracted along the pivot arm to keep the same object engaging feature in contact with the object being traced.
At each of a plurality of rotational positions, the amount of pivoting and the amount of translation (extension or retraction) are recorded. When the rotational position is combined with the amount of pivoting and the amount of translation, a three-dimensional vector is provided for each rotational position. The three-dimensional vector is represented by polar coordinates (Theta, Phi, Beta), wherein Theta represents the rotational orientation about an axis of rotation, Phi represents the pivot angle of the pivot arm, and Beta represents how far the object engager has been extended.
Most edging apparatuses, however, are configured to accept data in cylindrical format, not polar format. There is consequently a need in the art for a system capable of converting trace information in polar format into trace information in cylindrical format, the latter being more compatible with existing edging apparatuses.
Another feature of the exemplary pivot-based tracer described in the aforementioned patent application is a clamp which holds the object (i.e., the eyeglass frame, the lens, or the lens pattern) in a vertical or near-vertical orientation during the disclosed tracing process. As disclosed in that application, the vertical or near-vertical orientation allows eyeglass frames which have a wrap-around feature (e.g., a curved temple portion which wraps around the face of the wearer) to be traced without gravity causing the object engager to "fall out" from the groove which holds the lens in the eyeglass frame.
Yet another benefit of the vertical or near-vertical orientation is that it facilitates viewing of the eyeglass frame's engagement with the clamp from the operator's natural line of sight. It also facilitates use of a more natural and comfortable arm movement when placing the object being traced in the clamp.
The vertical or near-vertical orientation, however, causes the magnitude of the force exerted by the object engager against the object being traced to vary as a function of the rotational orientation of the object engager. When the object engager traces the lower part of a lens mount in the eyeglass frame, for example, gravitational forces add to the biasing force toward the lens mount. By contrast, when the top of the lens mount is being traced, gravity counteracts the biasing force toward the lens mount.
The opposite is true during tracing of the lens or lens pattern. When a lens or lens pattern is traced, the tracing is performed around the external circumference, as opposed to the internal circumference. Gravity therefore tends to pull the object engager away from the lens or lens pattern when the bottom, not the top, of the lens or lens pattern is being traced. Likewise, when the top of the lens or lens pattern is being traced, gravity urges the object engager toward the object being traced.
At the nine-o'clock and three-o'clock orientations (the 180 degree and 0 degree tracing positions), the gravitational force on the object engager is orthogonal to the biasing force and therefore does not contribute to or counteract the biasing force.
During the tracing operation, it is desirable to apply a more constant force against the lens mount, lens, or lens pattern. Variations in the force applied against the lens mount, lens or lens pattern can cause inaccuracies in the trace data. In extreme cases, the variations which contribute to the biasing force might be strong enough to slightly overcome the clamping force and cause movement of the object being traced. Likewise, the variations which counteract the biasing force may be enough to cause the object engager to become disengaged from the lens mount, lens, or lens pattern.
There is consequently a need in the art for a control and/or data acquisition system for a pivotally actuated tracer, which system is capable of providing compensation for the effects of gravity on the tracer when the tracer holds objects to be traced in a vertical or near-vertical orientation.